WASHINGTON (Reuters)
— You can forget about the
birds and the bees. If you really want to learn how babies are made,
you need to know about Juno and Izumo.

Fertilization takes place when an egg cell and a
sperm cell recognize one another and fuse to form an embryo. But how
they recognize each other in order to hook up had remained a
mystery.

Researchers said on Wednesday they have identified a protein on the
egg cell's surface that interacts with another protein on the
surface of a sperm cell, allowing the two cells to join.

This protein, dubbed Juno in honor of the ancient Roman goddess of
fertility and marriage, and its counterpart in sperm,
named Izumo after a Japanese marriage shrine, are essential for
reproduction in mammals including people, they said.

This new understanding of the role of these two proteins could help
improve the treatment of infertility and guide the development of
new contraceptives, the researchers said.

"By identifying this interaction between Juno and Izumo, we now know
the identity of the receptor proteins found on the surface of our
father's sperm and our mother's egg that must interact at the moment
at which we were conceived," said Gavin Wright of the Welcome Trust
Sanger Institute in Britain, one of the researchers in the study
published in the journal Nature.

The researchers are now screening infertile women to try to
determine whether problems with the Juno receptor are to blame.

"It is remarkable that about 20 percent of infertility cases have an
unexplained cause," said Enrica Bianchi of the Sanger Institute,
another of the researchers.

"We are now asking whether Juno is involved in these cases of
unexplained infertility," Bianchi added.

Wright said that if defects in the Juno receptor are in fact
implicated in human infertility, a simple, non-invasive genetic
screening test could be developed to identify affected women.

"This then would allow us to guide the fertility treatment,"
Wright said, letting affected women proceed directly to a procedure
called intracytoplasmic sperm injection involving direct injection
of sperm into an egg obtained from in vitro fertilization.

Japanese researchers identified the sperm cell's Izumo protein in
2005, but the identity of its counterpart on the egg cell had
remained elusive. The Sanger Institute researchers made an
artificial version of Izumo to try to find an answer, and found that
it interacted with Juno to initiate fertilization.

They then developed mice that lacked Juno. The females of these
mice were infertile because their egg cells did not fuse with sperm.
The Japanese researchers earlier had shown that male mice lacking
the Izumo sperm protein were also infertile.

In the new study, the researchers detected a quick loss of the Juno
protein from the egg's surface after fertilization. They said this
may account for how a fertilized egg blocks out additional sperm
cells, preventing formation of embryos with more than one sperm cell
that would not be viable.